1. Field of the Invention
The present invention relates to a lens driving device, and more particularly to a lens driving device for focusing and zooming the lens of a still camera, a video camera or the like by means of a motor.
2. Description of the Related Art
A conventional motorized lens driving device uses a plurality of on/off switches in combination as operating members. The lens is driven at one or two setting speeds, three at the most. In order to increase the number of the lens driving setting speeds, it is required to increase the number of on/off switches. Increasing the number of the switches is unsuitable for equipment, such as a camera, because space is limited.
In addition, there is a device for controlling the driving speed in accordance with the amount of operation or speed of operation of a rotating member. Moreover, there is a device for controlling the driving speed in accordance with the amount of operation of a sliding contact that slides on a resistance body. However, these devices require large space and are thus unsuitable for cameras.
Therefore, various types of devices have been proposed recently which use a pressure sensitive resistance element and control the driving speed in accordance with pressure (pressing force) applied thereto (refer to Japanese Unexamined Patent Publications Nos. 47-37733, 55-133021, 61-144801, 62-112111, and 62-155791). These devices permit stepless control of the driving speed with only space for the conventional switches.
However, the conventional devices using a pressure sensitive resistance element do not match the user's operative sense because a one-to-one correspondence exists between driving speed and pressure. That is, the pressure versus resistance characteristics of the pressure sensitive resistance element is such that a change in resistance is great in the initial stage of application of pressure, and the greater the pressure becomes, the smaller the change in resistance becomes. In the case of fine adjustment, on the other hand, the operator will perform careful operation while lightening pressure. In this case, it is desired that the rate of change in driving speed with respect to change of pressure be low.
That is, with the conventional driving devices, the rate of change in driving speed with respect to change of pressure will become great when pressure is low, in other words, when the operator makes fine adjustment, failing to give camera users a good operative sense.